The present invention relates generally to MR imaging and, more particularly, to a method and system of determining motion in a region-of-interest directly from MR data acquired from the region-of-interest independent of k-space trajectory of a k-space filling scheme carried out to sample the region-of-interest.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Any type of subject motion—cardiac, respiratory, or other body motion—during MRI may introduce image artifacts that affect the quality of an image and, ultimately, diagnostic utility of the scan. A number of techniques have been developed to reduce motion artifacts in MRI. One class of techniques synchronizes data acquisition with motion using breath-holding, respiratory triggering, and/or cardiac gating to “freeze” subject motion. Another class of techniques corrects data acquired in the presence of motion. Despite the use of these techniques, however, motion remains one of the primary impediments to diagnostic image quality.
Most known motion artifact reduction techniques require the ability to measure subject motion during the scan, either by external physical tools (e.g. respiratory bellows, electrocardiogram (ECG), pulse oximetry, and the like) or by interleaving extra gradient pulses into the pulse sequence (e.g. navigator echoes)—both of which impose additional time, cost, and complexity to the scan and scan system. Furthermore, external physical measurements are typically indirect, error-prone measures of motion that may not accurately reflect true motion in the region-of-interest. For example, respiratory bellows measure only the anterior-posterior component of breathing motion at the abdominal surface, while the ECG measures electrical rather than mechanical cardiac activity. While breath-holding can be used to minimize respiratory motion artifacts and does not require the ability to measure motion, it limits image quality, suffers from drift and position inconsistencies, and is often impractical in severely ill or pediatric subjects.
Other known motion assessment techniques are k-space trajectory limited. That is, one known technique determines motion in a region-of-interest directly from MR imaging data acquired from the region-of-interest. That is, this technique determines motion from spatially encoded data, rather than data acquired without spatial encoding. In this regard, this technique is severely limited in its applications. Simply, the technique is applicable only with spiral and radial sampling. As such, the technique cannot be used for MR scans in which resonance is sampled and k-space filled using non-spiral or non-radial k-space trajectories. As there are a number of MR imaging techniques that do not rely upon a spiral or radial k-space trajectory, this known technique, and others that are k-space trajectory limited, is frequently inapplicable.
Other proposed “self-navigated” techniques sample additional echoes that are generated without phase encoding. The data associated with these additional and fully-sampled echoes is then analyzed to determine motion in a region-of-interest. Since “extra” echoes must be induced and then sampled, such a technique can significantly lengthen scan time and increase the memory and processing requirements of an MR scanner.
It would therefore be desirable to have a system and method capable of more direct, efficient, and accurate measuring of motion in a region-of-interest that is independent of k-space trajectory such that motion artifact reduction techniques may be applied with greater success in many MR applications and without increasing scan time.